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Maximum current,

Discussion in 'Electronic Design' started by Danny, Oct 4, 2003.

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  1. Danny

    Danny Guest

    Hi

    Could anyone tell me how to calculate the maximum current that a wire
    carry, presumably a formula for the heating effect in relation to its
    cross-sectional area? As also i would assume it brings in the thermal
    coefficient.

    Dan
     
  2. For high currents it's not only heating.

    If you got two wires next to each other with opposite currents, the
    magnetic field can have enough force to split it into two separate wires.

    skin effect, the electrons move on the surface of the wire, at higher
    frequencies this effect is stronger.

    Once a wire starts to heat, it will have more resistance, if the current
    doesn't get lower when resistance get higher (is you circuit a current
    source?) you will reach the point of no return right after the wire starts
    to heat.

    So a "complete formula" is kind of hard, thermal resistance of your
    isolation and ambient temperature also play a part.

    If you could define your circuit, and define the conditions, it's easyer to
    make a formula.

    Cheers,
    Jeroen.

    joenix at gmx dot net
     
  3. Danny

    Danny Guest

    Hi

    Well it is a question build upon many different circuit ideas, not
    really a specific one, they are all, of sort, 12V automotive
    applications, which currents upto about 10A at maximum, and as low as 500mA.

    So if i needed say a wire to run a 5A bulb at 12 V, what clasification
    of wire would i need. Is there like a rule of thumb that most people use
    to classify current against wire type (like a table of ideal thicknesses
    etc)?

    Dan
     
  4. Take a look at this site.
    fp
    http://www.powerstream.com/Wire_Size.htm
     
  5. For only 10 amps DC, I wouldn't worry too much, just take a wire of 1mm^2
    and your're done.
     

  6. Tables showing ampacity for wire sizes shouldn't be taken too literally, but
    to give you an idea of what might be reasonable you might play around with
    some AWG wire calculators like this one:

    http://www.geocities.com/CapeCanaveral/Lab/9643/awg.htm

    In many low voltage applications (of which automotive is a good example) the
    wire current density is more limited by total voltage loss rather than the
    maximum dissipation capability of the wire. As a result you as the
    engineer get to decide how much voltage loss is appropriate in any given
    application to determine maximum wire resistance and then wire size.
     
  7. Ian Stirling

    Ian Stirling Guest

    Also, depends on in what condition you want the wire afterwards.
    You can pass many thousands of amps through a wire thinner than a
    hair, but after a few nanoseconds it'll be expanding outwards at several
    kilometers per second...
     
  8. Ian Stirling

    Ian Stirling Guest

    Also, you probably don't want to spec wires that can safely pass only
    500ma, in an automotive enviroment.
    Unless they are very well supported, they will be rather fragile, especially
    when installing.
    1mm^2 is a pretty good minimum.
     
  9. Ben Bradley

    Ben Bradley Guest

    What you probably want is to figure out the lowest voltage you find
    acceptable at the bulb (maybe 11.75V when the wire is connected to
    12V) and choose the wire based on that. This example gives a .25V drop
    across the wire. At 5A this gives a resistance of .25V / 5A = 0.05
    ohms. Choose a wire that will have that resistance or less at the
    length (both directions, unless you use the chassis as the return)
    from the source to the bulb.
     
  10. N. Thornton

    N. Thornton Guest


    Hi

    Car electrics aren't 12v, they are nearer 14v.
    Also a fraction of a volt can make a significant difference to light
    output on a [nominally] 12v system, and that of course affects safety
    and driver comfort. So seeing what wire you can get away with probably
    isn't the way to do it in this case.

    IIRC around 3mm2 is commonly used for headlights.


    Regards, NT
     
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